There is a propelling system for a vehicle, which is conventionally known from Japanese Patent Application Laid-open No. 5-340241, wherein a heat energy of an exhaust gas from an engine is converted into a mechanical energy by a Rankine cycle system mounted on the vehicle, and the mechanical energy is united to a driving force from a crankshaft of the engine to assist in the traveling of the vehicle.
It should be noted here that in the conventional vehicle, an accelerator pedal operated by a driver and a throttle valve of an engine are connected mechanically to each other by a cable, so that an accelerator opening degree and a throttle opening degree are matched to each other at 1:1. For this reason, when the driving force from the engine and the driving force from the Rankine cycle system are united to each other to drive the driven wheel, the following disadvantages arise due to a delay of response of the Rankine cycle system:
As shown in FIG. 10, in the conventional vehicle including the accelerator pedal and the throttle valve connected mechanically to each other by the cable, the throttle opening degree is varied substantially without a response delay in proportion to the accelerator opening degree, and the engine output is varied substantially without a response delay in proportion to the throttle opening degree. However, it is difficult for the output from the Rankine cycle system operated by the exhaust gas from the engine to follow the accelerator opening degree accurately, because the change in heat energy of the exhaust gas from engine has a response delay of about 0.5 second (a first-stage response delay) with respect to the change in accelerator opening degree due to an influence such as an abatement of heat in an exhaust port, and the change in heat energy of vapor generated in an evaporator has a response delay of about 5 seconds (a second-stage response delay) with respect to the change in heat energy of the exhaust gas due to an influence such as a thermal capacity of a heat transfer pipe.
As a result, immediately after the driver has stepped on the accelerator pedal, the output from the engine is increased immediately, whereas the output from the Rankine cycle system is increased with a response delay. For this reason, a total output resulting from the addition of the output from the engine and the output from the Rankine cycle system to each other is temporarily deficient, resulting in the arising of a disadvantage that the driver feels a sense of incompatibility (see a portion indicated by a). Immediately after the driver has returned the accelerator pedal, the output from the engine is decreased immediately, and the output from the Rankine cycle system is decreased with a response delay. Therefore, a total output resulting from the addition of the output from the engine and the output from the Rankine cycle system to each other is temporarily excessive, resulting in the arising of a disadvantage that the driver feels a sense of incompatibility (see a portion indicated by b).